The present invention relates to a starter system for a gas turbine engine, and more particularly to an independently controllable starter system and fuel system for an auxiliary power unit.
In a conventional start system for a gas turbine engine, for example, one used in an auxiliary power unit, a start sequence that coordinates engine speed, ignition and fuel delivery is required to achieve a reliable start. Conventionally, a dedicated starter motor or a starter-generator, is drivably coupled to the gas turbine engine and is operated to produce rotation thereof. As the starter accelerates the engine, a fuel delivery pump driven by a gearbox attached to a rotor of the gas turbine engine provides fuel flow thereto. Igniters are then actuated to effect ignition in a combustor of the engine. Upon successful ignition, and once the engine has reached a self-sustaining speed, the starter is disengaged.
For successful ignition, engine speed and fuel delivery must be coordinated to provide an air fuel mass mixture at the igniter which is capable of sustaining combustion. The range of engine speeds at which starting is most likely to occur is referred to as the xe2x80x9clight off windowxe2x80x9d and typically ranges from 5%-20% of rated engine speed. If the starter accelerates too quickly through the light off window, the gearbox driven fuel pump may have insufficient time to deliver fuel into the fuel system, thereby resulting in failed ignition.
Typical gearbox driven fuel pumps are sized to provide fuel at relatively low pump speeds for filling fuel lines and atomization at the igniters for light off. Disadvantageously, the low speed flow requirement results in a rather large fuel pump which requires recirculation during normal operating conditions. Recirculation creates complex thermal conditions during operation which further complicates the system. This is particularly disadvantageous to the design of a compact APU system
Further, a high power starter is often necessary to ensure engine start on a cold day and/or when a battery supply is low. However, the same starter will be capable of very fast engine acceleration on a warm day with a fully charged battery which results in reducing the light off window. A gas turbine igniter typically operates at a rate of 3 to 10 discharges per second. Rapid acceleration through the light off window therefore limits the number of igniter discharges occurring within the window, which may further reduce the likelihood of successful ignition.
The high torque capability of the starter may also exceed the maximum rating of the gearbox which may not be safely operated during the start-up procedure. Thus, while the electrical loads could be safely operated during steady state operation, the starter motor portion of the generator could not operate safely during the start-up procedure. Providing a gearbox for the higher torque rating would result in added weight and prohibitive cost to retrofit existing aircraft or new aircraft using this same engine.
Accordingly, it is desirable to provide a starting system for an gas turbine engine which prolongs the duration of the light off window and provides a controlled dwelling point to ensure start reliability and minimize thermal shock.
The starting system for a gas turbine engine according to the present invention provides a controlled starter system and a controlled fuel system. Each system includes an independent controller which operates each respective system. The controlled starter system generally includes a brushless DC starter motor which is directly connected to a rotor of the gas turbine engine to provide torque thereto. The fuel system generally includes a pump motor which communicates with the fuel system controller to drive a fixed displacement fuel pump at variable speeds to supply fuel to the gas turbine engine independent of engine speed. As the fuel pump is independently controlled, a fuel recirculation system is avoided or minimized. System weight and complexity and power consumption is thereby decreased and undesirable thermal conditions are avoided.
In operation, a start sequence is initiated by a start command such that the starter system controller commands the starter motor to provide a controlled torque to the rotor of the gas turbine engine rotor. The controlled starter motor is accelerated along an acceleration profile to a dwelling point. Concurrently with the starter motor spinning up the gas turbine engine rotor, the fuel system controller commands the pump motor to drive the fuel pump to establish adequate fuel pressure for atomization and light off. As the fuel pump motor and starter motor are independently controlled and coordinated, the starter motor maintains the gas turbine engine at the predetermined dwelling point until ignition occurs. In other words, the light off window does not close because the controlled starter motor is varied to maintain the gas turbine engine at the dwelling point until ignition occurs.
Once ignition is detected, the pump motor is controlled such that the fuel pump provides a fuel flow to smoothly accelerate the gas turbine engine. The starter motor also provides a controlled amount of assisting torque during acceleration of the gas turbine engine. The assisting torque assures optimization and coordination of the fuel schedule and gas turbine engine acceleration to minimize thermal transients to the turbine rotor which thereby increases operational life.
The fuel pump for the present invention is preferably a brushless DC motor having an external stator winding within which is a rotatably mounted a tubular armature. A filter is mounted for rotation within the armature. Flow through the filter is driven by the centrifugal force of the rotating armature which eliminates the requirement for a separate boost pump.
The present invention therefore provides a starting system for a gas turbine engine which independently coordinates and controls operation of a starter motor and fuel pump to prolong the duration of the light off window and provide a controlled dwelling point to ensure start reliability and minimize thermal shock.